Crews were dispatched to the fire at an apartment in a three-unit building at 425 S. 23rd St. around 5:30 p.m. Wednesday, according to Richmond fire officials.
Firefighters arrived to find a fire burning just inside the front door of a second-floor apartment and a badly burned man, fire officials said.
He was pronounced dead at the scene.
A Contra Costa County coroner's deputy said the coroner's office was still working to identify the man as of Friday morning. Dental records were being reviewed to help determine his identity.
Fire officials said crews quickly extinguished the fire on Wednesday and were able to contain it the area around the home's front door. No neighbors were displaced.
Richmond fire Marshal Terry Harris said fire investigators are still interviewing neighbors and other witnesses to determine what caused the deadly blaze.
Harris said fire officials are "investigating all possible causes" including suicide.
Richmond police, who were also called to investigate the fire, initially said it may have been caused by an electrical problem, but Harris said that has been ruled out as a potential cause.
"We should have something solid at least within two weeks," he said.